The present invention relates to a desulphurisation process. More particularly, the present invention relates to the removal of sulphur compounds such as hydrogen sulphide from process streams. Most particularly, the present invention relates to the removal of sulphur compounds from hydrocarbon streams.
Hydrocarbon process streams often comprise a significant quantity of sulphur compounds. For example, a gaseous hydrocarbon feed may comprise in excess of 50 ppm by volume expressed as equivalents of hydrogen sulphide.
It is generally desirable to remove these sulphur compounds from the feed or at least to reduce them to a low level, for example to a level in an amount of about less than 5 ppm by volume. Indeed, more recently, there has been a demand for the sulphur content to be reduced to much lower levels, for example of the order of 1 ppm by volume or less.
One method of sulphur removal that has been used is to contact the feed with a bed of an absorbent catalyst such as zinc oxide which will remove some of the sulphur. Zinc oxide generally has a low capacity for sulphur at reduced temperatures and therefore the contact between the feed and the zinc oxide is generally conducted at elevated temperature for example at from about 260° C. to about 450° C.
Further the quantity of sulphur that escapes the zinc oxide bed is related to the quantity of sulphur that has already been absorbed into the zinc oxide bed. Thus a bed with 10% sulphur absorbed onto it might produce an exit gas with 1 ppm sulphur in it, but when the absorbed sulphur has increased to 20%, the exit gas might contain 10 ppm sulphur.
The bed of zinc oxide will generally require regular replenishment and it is therefore general practice to operate the desulphurization process with two beds located in series in positions (A) and (B). When replenishment is required, fresh zinc oxide is generally loaded in the bed in position (A) which is then switched such that it is in position (B). This means that the bed with the oldest catalyst is contacted with the feed first. The bed which was originally in position (B) is moved to position (A) where it will continue to operate until analysis of the exit stream from bed B suggests that replenishment is required again. The movement of the beds between positions (A) and (B) is usually carried out by switching flow in connecting pipework using valves.